Plastic and nonmetallic article shaping or treating: processes – With step of making mold or mold shaping – per se
Reexamination Certificate
2000-11-29
2003-06-10
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With step of making mold or mold shaping, per se
C264S328700, C264S334000
Reexamination Certificate
active
06576177
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a plastic gear molding system and method, in particular relates to a plastic gear molding system and method capable of preventing from partial shrinkage during its molding process.
2. Discussion of the Background
It is well known in a transmission apparatus to have at least one plastic gear that transmits a rotation force of a driving motor to a driven member. For example, in an image forming apparatus such as a copier, a printer, a facsimile and a multi-functioned machine having a plurality of functions, a rotation force of a driving motor is generally transmitted to an image carrier through a driven member contacting the surface of the image carrier or the like for forming a toner image on the surface of the image carrier during its rotation. The conventional plastic gear may also be used in a duplicator, a camera, a video deck and a compact disk player and so on to transmit a rotational force to a driven member thereof.
In recent years, such plastic gears have tended to be rotated at relatively high speeds and so have been subjected to higher external forces. Since the conventional plastic gear is simply constituted by a hub, a gear ring and a web whose ends connect the hub and the gear ring, it has been difficult to meet the necessary level of rigidity and strength required for the plastic gear. It is of course possible to increase both the rigidity and the strength to meet the prescribed level if both a thickness and a size of the background gear are increased.
However, this is costly and a transmission apparatus unavoidably becomes bulky. To increase the rigidity, a plurality of ribs may be symmetrically integrally mounted on both front and rear surfaces of the web in a manner such that each one edge connects with the hub and each another edge connects to the gear ring. However, a diameter of such a plastic gear generally varies during its molding process due to the so called a shrink phenomenon of the plastic. As a result, a peripheral speed of the plastic gear periodically changes when it rotates, and accordingly unevenness of the rotational speed of the driven member may arise.
The present inventors have determined that the shrink phenomenon occurs for the reason hereinbelow explained in detail referring to
FIGS. 10 through 12
. A background plastic gear
14
includes a cylindrical hub
25
disposed as a core portion thereof and is supported by the shaft
15
illustrated in FIG.
2
. The plastic gear
14
further includes a gear ring
27
having substantially concentric with the hub
24
and having a larger diameter than the hub
24
, which is disposed at of the hub
25
.
The gear ring
27
includes a plurality of gear teeth
26
on an outer circumferential surface thereof. The plastic gear
14
further includes a web
28
constituted by a circular plate whose ends integrally connect the hub
24
and the gear ring
27
. A plurality of ribs
28
A and
28
B are each integrally formed respectively on front and rear sides of the web
28
. Each of the plurality of groups of the ribs extends in a radial state from the hub
24
to the gear ring
27
.
The ribs
29
A formed on a front surface of the web
28
are arranged at a prescribed angular interval around the hub
25
. The ribs
29
B formed on a rear surface of the web
28
are arranged in a same way as the ribs
29
A. Each of the ribs
29
A and
29
B is symmetrically disposed at both the front and rear surfaces of the web
28
. As a result, a perpendicular cross section of the web
28
intersects the cross sections of both the ribs
29
A and
29
B as illustrated in FIG.
7
. Since a partial shrinkage phenomenon occurs at each of portions of the plastic gear
14
where the ribs
28
A and
29
B are disposed at same angular positions on the front and rear side surfaces of the web
28
, during a cooling process of molding, diameters of these portions decrease to be less than that of other portions.
When producing a gear made of a metal by cutting a metal material, such a partial shrinkage phenomenon, of course, does not occur. Such a partial shrinkage phenomenon may occur only in a case that a pair of ribs
28
A and
29
B are disposed at same angular positions on the front and rear side surfaces of the web
28
. The eccentricity of a gear periphery of the conventional plastic gear that includes a pair of six pairs of ribs
29
A and
29
B respectively formed on the front and rear side surfaces
28
A and
28
B of the web
28
is illustrated in FIG.
14
. As there shown, the diameter of the gear edge circle remarkably changes six times corresponding to the number of the ribs. As a result, the rotational speed of the plastic gear varies six times; thereby unevenness of the rotation speed arises when the conventional plastic gear rotates.
A possible cause of the change in rotational speed of the plastic gear is explained below. A portion of the gear ring
27
and gear teeth
26
A,
26
B and
26
C each mounted on the circumference of the gear ring
27
are typically illustrated in FIG.
15
. As there shown, ends of the ribs
29
A and
29
B are connected to the same portion (shown enlarged for ease of illustration) of the gear ring
27
between the tooth
26
B and
26
C. A space between the teeth
26
B and
26
C is illustrated larger than actual for an explaining purpose.
The portion of the gear ring
27
between the teeth
26
B and
26
C is more indented toward a rotational center of the plastic gear than other portions thereof, since the partial shrink occurs when the plastic gear is molded. Thus, the tooth
29
A positioning at a left side of the ribs
29
A and
29
B inclines on the right and the tooth
29
B positioning at a right side of the ribs
29
A and
29
B inclines to the left as illustrated in FIG.
15
.
A gear
26
D meshes with the plastic gear
14
as illustrated in FIG.
15
. If a pressure angle at a gear connecting portion at which a gear tooth
26
D of another gear meshes with the gear tooth
26
A is &agr;
0
, a pressure angle &agr;
1
, of the gear tooth
26
B inclining to the right is larger than &agr;
0
, A pressure angle &agr;
2
of the gear tooth
26
C inclining on the left is smaller than &agr;
0
.
If angular velocities are &ohgr;
0
, &ohgr;
1
and &ohgr;
2
correspond to gear portions having the angles of &agr;
0
, &agr;
1
, and &agr;
2
.
The larger the pressure angle, the smaller the angular velocity and the smaller the pressure angle, the larger the angular velocity. Thus, the following relation is established around the ribs
29
A and
29
B.
&ohgr;
1
<&ohgr;
0
<&ohgr;
2
Thus, when ribs
29
A and
29
B extend in a radial state, for example, from the rotational center of the gear and are each disposed in a same angular interval, a rotational speed of the gear periodically varies when the plastic gear rotates.
Further, a rotational speed of the conventional driving motor
10
generally varies once per one revolution thereof. Thus, a rotational velocity of the PC drum
1
remarkably changes at a prescribed timing, if a frequency of a change in rotational speed of the conventional driving motor
10
is substantially coincident with that of the plastic gear
14
. This is because, cylindrical peaks due to the change in rotational speed of the driving motor
10
and that due to the plastic gear
14
coincide with each other. As a result, unevenness of a toner image (so called the jitter) arises on the surface of the PC drum
1
, and the image quality is inferior.
For example, if the driving motor
10
rotates at 1,800 rpm, a frequency of a change in rotational speed is 30 Hz (obtained by dividing 1,800 rpm by 60 seconds). If the number of teeth of the output gear
13
of the driving motor
10
is ten, a number of teeth of a plastic gear
14
that meshes with the output gear is seventy, and a number of ribs
28
A and
28
B mounted on each of the surfaces of the web
28
of the plastic gear
14
is seven, a frequency of a change in rotational speed of the gear
14
becomes 30 Hz, as is obtained by the following formula.
1800 rpm&ti
Kuhns Allan R.
Ricoh & Company, Ltd.
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